Lithium based batteries have grown in popularity over lead-acid batteries due to their relatively small size and weight compared to lead-acid batteries of similar capacity. Although common in small devices such as cellular phones and cameras, lithium based battery use is becoming more common to power larger loads such as electric cars.
An enclosed battery module, such as those used to start internal combustion engines, typically consist of multiple battery cells coupled together to meet the power requirements of the application. For example, in a battery application requiring 12 VDC, a battery module may consist of four battery cells of at least 3 volts electrically coupled together in series to sum the cell voltages for the battery module. Several lead acid battery cells may be “stacked” or connected electrically in series by a simple conductor to sum the individual cell voltages to create a lead acid battery module that meets the vehicle power requirements. When the vehicle is started, current is drawn from the lead acid battery module to power the starter motor and start the internal combustion engine. Starting operations result in a significant power drain that replenished by an alternator that recharges the battery module to power engine electronics and future vehicle starts. This simple but effective system however is not sufficient for all battery technologies. Lithium-Ion batteries for example present special challenges for vehicle installations and in other similar applications where the re-charge voltage and current are less than constant. Lithium-Ion batteries have low tolerance for overcharge and should be charged with controlled voltage and current to deter degradation or damage to the cells.
Generally, equalization of the battery cell charges during charging operations avoids battery cell over-charge. Lithium-Ion battery systems therefore commonly include smart circuitry that compares and balances the charge on each Lithium-Ion battery cells to prevent overcharge of individual battery cells during charging operations. One common prior art solution includes battery cell balancing circuitry that is coupled to the individual battery cells and that permits controlled discharge of individual battery cells. The battery cell balancing circuitry samples and compares each of the individual battery cell voltages to each of the other battery cell voltages (or a reference voltage) and balances or discharges individual battery cells having charges that exceed the lowest charged battery cell. This technique typically includes measuring each battery cell charge and producing corrective charging feedback in a control loop that causes a discharge of individual battery cells to match the charge on other cells.
U.S.2009/0087722 to Sakabe et al. (“Sakabe”) discloses an example of a battery module as described and includes the charge balancing circuitry described. Sakabe discloses a vehicle power supply system battery module comprised of the accumulated battery cell potentials from three distinct but identical battery cell subunits that are hard-wired electrically in series to deliver power from all of the battery cells. Similarly, the Sakabe battery module is charged by current that is delivered to all of the serially connected battery cells. Sakabe's battery module however would be non-functional should any of the Sakabe battery cells or subunits fail.
A vehicle battery system with a backup capability is described in U.S.20120235642 to Mao et al. (“Mao”) wherein a battery module is comprised of subunits having a battery cell and a super-capacitor connectable in parallel as back-up or alternative power. In certain conditions, the battery cell charge may be insufficient to start a vehicle but have enough residual charge to charge a super-capacitor that can discharge very quickly and start a vehicle, or be used when the battery cell temperature is too cold or too hot and unable to deliver sufficient charge to start a car engine. As an example, Mao discloses a number of battery cells connected electrically in series to accumulate a battery module potential that is connected to an alternator to maintain the charge on the battery cells. An ultracapacitor pack comprised of several ultracapacitors electrically in series is then connected in parallel with the battery cells to provide a short and high current for starting the vehicle. Again however, if any of the battery cell(s) or super-capacitor(s) falters, the Mao battery module performance suffers or fails altogether.
Accordingly, there remains a need for improvements in battery module architectures and management strategies to facilitate use of lithium-based batteries chemistries in applications where lead-acid based battery use currently dominates.